Worldwide water shortages and the expense of cleaning up polluted water has resulted in an increasing need to dispose of toilet waste without the use of water or centralised sewage processing.
Toilets which do not use water and which do not depend on central sewage processing fall into four types--incinerating toilets, sterling toilets, composting toilets and lined toilets. Incinerating toilets use an external source of energy to heat the waste, evaporate all the liquid waste and burn the remaining solid waste. Disadvantages are the high initial cost, the high operating cost and the use restriction imposed by the incinerating cycles. Similarly, sterilizing toilets use external energy to evaporate urine and heat the feces to kill the bacteria but require the inconvenience of frequent manual clean out for disposal elsewhere. Composting toilets store the waste in an aerated chamber and use aerobic bacteria to digest or oxidise the waste over a long time period. Composting toilets are bulky and relatively expensive installations which need frequent manual attention. Lined toilets surround the waste in plastic and transport it to a storage container for eventual manual removal and separate disposal elsewhere.
None of the foregoing waterless toilets perform their function as conveniently and with as little user attention as a conventional water flushing toilet. However, the water flushing toilet does not have to actually dispose of the waste. It uses clean water to move the waste to a remote location where more expensive disposal processes are undertaken. In many parts of the world this is no longer an economically viable procedure.
Most toilets which combine solid and liquid waste together cannot take advantage of the fact that liquid toilet waste containing urine and any flushing water is much easier to dispose of than solid toilet waste containing feces, paper, diapers, fibrous sanitary items and potentially harmful pathogens. Urine which is normally sterile and constitutes the bulk of human waste is readily absorbed into soil or dispersed in waterways without environmental harm whereas solid toilet waste requires special treatment before it can be released into the environment.
To eliminate a health hazard it would be better to incinerate the solid toilet waste but the high water content and the low heating value of the waste makes this expensive and complicated.